WO-A-2005/108128 describes a roll control system for a motor vehicle which includes a hydraulic control circuit. The hydraulic control circuit includes a source of fluid pressure, a fluid reservoir, a pressure control valve, and a number of other valves, all of which control the fluid flow to, and the fluid pressure in, the chambers of hydraulic actuators. The source of fluid pressure is a pump which is either continuously driven by the vehicle engine, or is electrically driven. As shown, but not described, in WO-A-2005/108128, the hydraulic control circuit includes an attenuation hose positioned between the pump and the pressure control valve and the other valves. The attenuation hose is intended to reduce any noise, vibration or harshness effects caused, or created, by the pressure pulses from the pump. The attenuation hose may include complex design features (such as restrictions, spirals, specific expansion rates, interconnected hoses of differing lengths) which are intended to reduce pressure pulses at differing frequencies. However, the presence of the attenuation hose creates a significant pressure drop (in the range of 10 to 20 bar (1000 kPa to 2000 kPa)). This pressure drop has an adverse effect on power loss and power consumption for the pump, even when full pump flow is not required.
FR-A-2775024 and U.S. Pat. No. 5,492,451 both discloses hydraulic control circuits in which a valve is fluidly connected between a fluid reservoir and the fluid line connecting the pump outlet to the inlet of an attenuation means. However, in both of these examples, the valve is a pressure relief valve which has the sole purpose of acting as a safety device to prevent excessive fluid pressure reaching the hydraulic circuit.